Heavy duty relays are used when a very high inrush current between 40 A and 1 kA has to be produced. In the prior art, at this high inrush current, starter relays are used with a tungsten contact. For switching, firstly the tungsten contact is closed, so that the high current peak flows over it. Only after this an AgSnO contact closes which conducts the current in the contact point. In this embodiment it is avoided that, with closed relays, the current flows exclusively via the tungsten contact which has poor conductive properties.
The switching force is produced in the heavy duty relay by electromagnetic means, such as a switching current conducted by a coil. An electromagnetic force results therefrom which pulls a movable armature, the armature movement being transmitted to a changeover spring which is resiliently deflected from its resting position by the switching force. If the switching force is discontinued, the changeover spring springs back again into the resting position. The changeover spring is provided with a changeover contact which is pressed in the contact position against the normally-open contact which is arranged on a rigid support and produces a conductive connection. The compressive force is either produced by the resilient return force of the changeover spring or by the switching force. With a heavy duty relay with a tungsten contact and an AgSnO contact, the two changeover contacts are arranged on the changeover spring; corresponding normally-open contacts made from tungsten and AgSnO are associated with these contacts on a rigid support.
This construction is problematical in that the tungsten contact is very expensive due to the material costs and in that the tungsten is hard to machine due to its hardness. Moreover, the additional AgSnO contact is required. As a result, the solutions known from the prior art for switching high current in the region of between 40 A and 1 kA are complex and expensive.
A relay is disclosed in EP-B-0691667 with a normally-open contact arranged on a rigid contact support which is not deflected in the contact position. It has been established in tests that with the relay of EP-B-0691667 only a very low number of switching operations can be achieved at high currents, i.e. the relay is irreparably damaged after a number of switching processes which is too low to be practical.
A further relay for switching current considerably below 40 A is known. With this relay a fixed and a movable spring contact are constructed identically to one another in the form of a changeover spring and a normally-open spring contact and they are used mirror inverted in a plane in a base of the relay. Due to their identical construction the two spring contacts have, by necessity, identical spring stiffness, which at inrush currents of between 40 A and 1 kA leads to only a low number of switching operations, which in practice does not allow use for inrush currents over 40 A.